X-ray detectors are used in electron microscopes (e.g., scanning electron microscopes and tunneling electron microscopes), X-ray spectrometers (e.g., X-ray fluorescence spectrometers/energy-dispersive X-ray spectrometers), and other instruments to analyze the composition and properties of materials. An illumination source (such as an electron beam or X-ray source) is directed at a sample to be analyzed, resulting in emission of X-rays from the sample wherein the X-rays (X-ray photons) have energies which are characteristic of the atoms of the sample from which they were emitted. Thus, the counts and energies of the emitted X-rays can indicate the composition of the sample.
Traditionally, X-ray detectors utilized lithium-doped silicon detector (Si(Li)) detectors. These detectors are semiconductor detectors which generate a charge upon receiving X-rays, and thereby allow counting of X-ray photons and measurement of their energies. In an arrangement used in the NanoTrace™ detector, manufactured and sold by Thermo Electron Scientific Instruments (Madison, Wis., USA), the Si(Li) detector output was preamplified using a FET, i.e., a field effect transistor, and the signal at the FET output (drain) was then provided to a cascode amplifier and pulse processor. To keep the FET in its linear range, feedback was used from the FET output (drain) to the FET input (gate). While this arrangement worked well, it disadvantageously had a response which is rather slow by present standards—it could not accommodate X-ray photon counts of greater than approximately 60,000 counts per second—and additionally it required cooling to cryogenic temperatures for greatest accuracy.
In recent years, more advanced semiconductor detectors, such as Silicon Drift Detectors (SDDs), have become available, and these offer the possibility of far greater count rate measurement (a million counts per second or more) with lesser temperature control burdens (see, e.g., Iwanczyk et al., High Throughput High Resolution Vortex™ Detector for X-Ray Diffraction, IEEE Transactions On Nuclear Science, Vol. 50, No. 6, December 2003). Unfortunately, SDD's carry their own drawbacks, in particular the problem of varying detector response with x-ray illumination level: the SDDs, which often have an integrated FET provided for signal amplification purposes, have a gain which changes with voltage (Hansen et al., Dynamic Behavior of the Charge-to-Voltage Conversion in Si-Drift Detectors With Integrated JFETs, IEEE Transactions On Nuclear Science, Vol. 50, No. 5, October 2003). This results in variations in the measurements of detected X-rays, and in turn difficulties in interpreting their significance. For example, for a manganese K-alpha X-ray detected by an SDD, as the measured photon count rate goes from 10,000 counts per second (10 kcps) to 100 kcps, its measured energy can shift by 50-100 eV (a phenomenon known as peak shift), and resolution (the peak width on an intensity/counts vs. measured energy scale) can degrade by 10-30 eV. (Fiorini, C, A charge sensitive preamplifier for high peak stability in spectroscopic measurements at high counting rates, IEEE Transactions On Nuclear Science, Vol. 52, No. 5, October 2005; Niculae, A.; Optimized Readout Methods of Silicon Drift Detectors for High Resolution X-Ray Spectroscopy, 2005 Denver X-ray Conference, Colorado Springs, 5 Aug. 2005). As noted by Fiorini, feedback to the drain can help reduce peak shift (shift in the peak on an intensity/counts vs. measured energy scale), it does not solve the problem of resolution degradation. Peak shift can also be reduced by using feedback to control the source current, as described in European Patent Application EP 1 650 871 A1.
Another method for reducing degradation at high count rates is to use a “pulsed reset,” wherein the gate is reset through a diode after a predetermined period to some datum range or value of voltage(s). This method, which is discussed in the Niculae reference above, is in contrast to “continuous reset” methods, which use leakage current in the FET to discharge the gate (Bertuccio et al., Silicon drift detector with integrated p-JFET for continuous discharge of collected electrons through the gate junction, Nucl. Instr. & meth. A 377 (1996)).
Despite all of the foregoing methods, additional methods would be useful, since most of the foregoing methods do not significantly assist in reducing one or more of peak shift and resolution degradation, and in any event further reduction in peak shift and resolution degradation would be useful.